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We determined the THz absorption coefficient of a 5 mol% MgO:LiNbO3 crystal using synchrotron transmission measurements over the frequency range from 0.75 to 6 THz and temperatures from 37 to 300 K. A broad peak at 3.15 THz was observed at all temperatures, which we assigned to be a low-frequency mode associated with defect centers in the crystal that causes unexpected increased absorption. We estimate stimulated polariton scattering Stokes gain at varying temperatures. We found that the additional mode strongly affects the gain, which explains the difficulty of using MgO:LiNbO3 for THz generation beyond 3 THz. The implications of our findings for operating THz lasers based on MgO:LiNbO3 beyond 3 THz, including temperatures down to 77 K, are discussed.

We demonstrate a new, to the best of our knlowledge, cavity design for terahertz (THz) lasers based on stimulated polariton scattering (SPS). The design simplifies the angle tuning of these lasers, which require non-collinear cavity fields at fundamental and Stokes wavelengths to cross in an SPS crystal with adjustable crossing angle. A mirror shared by both the fundamental and Stokes cavities ensures stationary overlap of the fields within the crystal, with the angle between the fields tunable by adjustment of one axis of a single mirror. We demonstrate the design for an intracavity SPS laser using a rubidium titanyle phosphate (RTP) crystal, and achieve single-mirror tuning of the THz output in bands between 3 and 5.8 THz, with a maximum output of 78 μ W at 4.08 THz.

A semiconductor terahertz (THz) emitter based on an active i-/n-GaAs layer integrated on a porous silicon (PSi) distributed Bragg reflector (PSi-DBR) is presented. It is specifically designed for the use of a very thin GaAs film of thickness less than the penetration depth of the 800nm laser beam $(\sim 1\mu \mathrm{m})$ . The DBR acts as a reflecting substrate for the excess transmitted photoexcitation. Using a 550nm-thick GaAs, the novel design exhibited a 67% increase in peak-to peak THz signal compared to a similar GaAs on silicon (Si) substrate. The enhancement can be attributed to the increased optical absorption and multiple reflections in the active layer.